This document relates to sensing devices and sensing techniques and their applications in sensing and detecting various materials, including chemical and biological substances.
Plasmons are eigenmodes of collective density oscillations of quasi-free electrons or an electronic gas in metals and other materials under optical excitation. Surface plasmons can be generated by coupling photons and electrons at or near a surface of an electrically conductive material as surface plasmon polaritons (SPPs). In addition, surface plasmons can be excited or generated as localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures.
Some surface plasmon resonant (SPR) sensors use propagating SPP in sensing the refractive index change surrounding the metallic surface or nanostructure. One such sensor is the SPP sensor in the Kretschmann configuration which uses a high refractive index prism for evanescent coupling under a large incident angle to excite the liquid-metallic SPP wave on the liquid metal interface, and grating configuration, both reflection and transmission, which uses grating wavevector matching condition to excite propagating SPPs on the interface of liquid and metallic grating. In such SPP sensing devices, the excited electric field is distributed on an extended surface area, so no ‘hot spot’ or concentrated electric field exists.
Other surface plasmon resonant (SPR) sensors use LSPP in sensing the refractive index change surrounding the metallic surface or nanostructure. For example, one such LSPP sensor includes nanoparticle scattering and random nanohole transmission configurations, which utilize the nanoresonant excitations induced by the particle's size and shape to sense the refractive index change around the nanostructures. In many implementations of such LSPP sensors, only “hot spot” exists as nanoresonance itself; however, there is little electric field on areas where nanostructures are absent. In addition, the excitation efficiency of LSPP is low due to the nanostructure's size and small portion of phase matching condition.